Hope this is the right forum. I am a desing engineer (mostly digital) and I have a problem with an audio circuitthat I kinda designed.

I have a bridge amplifier using BD139-16 and BD140-16 (see simplified rough pic).
It's part of a bigger system and we have made 50 of these. They were all kinda fine when testing the distortion with an AP (audio processor).

Now, another larger batch was made and ALL of them have huge distortion when testing on the AP.

I looked into it today with a signal generator and oscilloscope and I found that the output stage (Headset load = ~100 Ohm) seems to burst into a VERY (YES VERY)!! high fequency oscillation at a certain level and fequency of input. I have attached a rough sketch of what I get on the scope. The osillation is visible in the top half of the output waveform.

I have tried everything I can think of and so has my collegue. We are not able to solve it at this stage and we are desperate.
The oscillation frequency is actually 69.65MHz (!!!!!!!)
It all occurs suddenly when the sine peak reaches around ~1V.

Removing the BD140 transitor solves the problem but then of course, I can't drive the full voltage swing required.
Removing the BD139 makes not much difference.

Touching anything (the op amp output etc) with a 10:1 scope probe, changes the output waveform and it changes the level at which the oscillations occurs.

Lowering the 47 R resistor by adding a 120R in parallel, stops the problem but again, does not allow for the full voltage swing, if lower impedance loads are used.

We have tried caps across the 27k, 47R, output to gnd (47nF gets rid of the HF part but then there is a step in the output waveform). we have also tried various other caps and ideas but nothing helps.

How the heck can such a circuit oscillate at 70MHz???? We suspect the LM833 and that it may not be suitable for a bridge amp like this but then what is the alternative in 8 pin smd?
The Vcc rail is +30V.

Maybe some op-amp / audio / analog expert out there can point me in the right direction?

This looks like a rather crude amplifier!
First thing you should try is to put that integrating (compensating) capacitor to the full loop, i.e. attach the end it to the output (the common emitters) instead of to the output of the opamp.
You could also do a lot better in terms of the zero crossing distortion by adding and biasing the diodes and putting emitter degeneration resistors to the final transistors instead of in series to the output:

An Oscillator is basically an Amplifier with "Positive Feedback", or regenerative feedback (in-phase) and one of the many problems in electronic circuit design is stopping amplifiers from oscillating while trying to get oscillators to oscillate. Oscillators work because they overcome the losses of their feedback resonator circuit either in the form of a capacitor, inductor or both in the same circuit by applying DC energy at the required frequency into this resonator circuit. In other words, an oscillator is a an amplifier which uses positive feedback that generates an output frequency without the use of an input signal. It is self sustaining.

Then an oscillator has a small signal feedback amplifier with an open-loop gain equal too or slightly greater than one for oscillations to start but to continue oscillations the average loop gain must return to unity. In addition to these reactive components, an amplifying device such as an Operational Amplifier or Bipolar Transistor is required. Unlike an amplifier there is no external AC input required to cause the Oscillator to work as the DC supply energy is converted by the oscillator into AC energy at the required frequency.LC Oscillator Tutorial

Don't shoot it down yet. I have attached the real schematic.
The crossover distortion is not bad as it is and the overall distortion is around 0.05% measured with the AP 70Hz- 10kHz (on the ones that do not have the problem).

I know about biasing and crossover distortion and I did originally have a current source bias network, similar to the ones you showed earlier. It worked fine but did not improve the overall quality/distortion enough to be worthwhile and thus we dunmped it again.

Please do not worry about the audio quality. It is actually quite good for this application.

I also cannot really change 100s of boards to a vastly different design right now.

The problem is the oscillations.

Did I mention 70MegaHz ? (Lower FM radio band)

None of the compnents (specially the LM833) should be capable to oscillate at such a high frequency but somehow they do.

I also know about Zobel networks and I made an attempt to add one as you can see from the 22R and 47n Restor /Cap on the output.

I have tried capacitors from emitter junction to input of opamp and NOTHING has made anyy difference.

I have also attached some scope captures for you. you can see the sinewave output at a level just before the oscillation and then as soon as it is just increased, the oscillation is present on the peak of the sinewave. As the level is increased further, the oscillation spreads over the sinewave but eventually becomes invisible as the level gets more and more increased. The oscillation amplitude seems to remain constant.

The only that I see that may make a difference is that capacitors C23,C24 should be in parallel with the feedback resistors R49,R50 and not connected to the output of the opamp as this is not compensating for the extra gain stage of the transistors.
Also you should also have your bypass capacitors going from V+ and V- to ground and not just bridged across the supply.

I will take a closer look as somehow I believe I see an oscillator configuration somewhere the way it is displayed.
what happens if you increase the value of C23 and C24 aswell?